The present invention relates to a method and apparatus for determining the flow resistance of acoustically absorbing material. More particularly, it relates to a method and apparatus for determining the flow resistance of fully assembled acoustically absorbing duct liners by using a sinusoidal acoustic pressure in the absence of grazing air flow parallel to the liner surface.
Single degree of freedom sound absorptive duct liners currently used on aircraft engines consist of a face sheet bonded to cellular material such as honeycomb which is itself bonded to a solid back sheet. The face sheet may consist of a perforated skin or a woven wire cloth material bonded to a perforated skin. The latter construction is currently termed a "linear liner" throughout the aircraft industry. The flow resistance of linear liners has shown a high degree of insensitivity to the presence of air flow along the face sheet surface of the liner. Such flow is present within engine ducts during aircraft operation.
The acoustic properties of a liner are greatly influenced by the face sheet resistance. The resistance of the face sheet of a linear liner is measured using conventional methods and apparatus by performing a DC (i.e. Direct Current or steady state) flow resistance measurement. The woven wire mesh side of the face sheet is subjected to a steady air flow normal to its surface. In many instances, the effect of perforate hole blockage due to the bond between the face sheet and cellular honeycomb is incorporated by performing the measurement on the face sheet/honeycomb construction without the presence of the back sheet. The ratio of pressure drop across the face sheet to steady air flow velocity measured upstream of the face sheet determines its DC flow resistance.
A similar type of measurement can be performed on a non-linear type construction, perforated face sheet without the woven wire cloth. However, the resistance of non-linear liners has been determined to be highly influenced by the presence of grazing air flow. The resistance measured for such a construction would be indicative of its behavior only in the absence of grazing flow.
The DC flow resistance for a given flow velocity and its variation with velocity (defined as the non-linearity) is used to insure compliance with quality assurance criteria.
The DC flow resistance measurement can only be made on the face sheet itself without the presence of the back sheet structure because the back sheet would interfere with the flow of air. The requirement of a DC flow resistance measurement on partially assembled liners impedes the manufacturing process and imposes restrictions on the number of bond cycles required for construction. Such restrictions make it highly advantageous to develop a procedure for evaluating flow resistance of fully assembled liners.
The principal object of the present invention is to provide a method and an apparatus for determining the flow resistance of a fully assembled liner including a face sheet, honeycomb core and back sheet.
An object of the invention is to provide a method and apparatus for determining the non-linearity of a fully assembled liner.
Another object of the invention is to provide a method and apparatus wherein data obtained concerning the acoustic absorbing properties of a liner can be used to determine the mass reactance of the face sheet of the liner.
Still another object of the invention is to provide an automated method and apparatus for determining flow resistance capable of being operated by assembly line production personnel.
A further object of the invention is to provide a method and apparatus for determining flow resistance and non-linearity at a liner production facility for the purpose of monitoring quality assurance criteria.
Yet another object of the invention is to provide a method and apparatus which supplies a detailed permanent record of all measurements performed on an assembled liner.
A further object of the invention is to provide a method and apparatus for determining flow resistance and non-linearity of an assembled liner installed on an aircraft so as to determine the degradation in resistance and non-linearity of the liner resulting from in-service usage.
A further object of the invention is to provide a method for compensating for measurement error introduced by such factors as honeycomb cell misalignment with the waveguide used for measurement, transverse propagation of acoustic energy through cell walls, the presence of water drainage holes extending into adjacent cells outside the boundaries of the waveguide and transmission of acoustic energy through the liner back sheet, when using acoustic oscillatory pressures instead of static pressures.